Solid state lights are increasingly replacing traditional incandescent and fluorescent lamps in many applications. Solid state lights have many advantages over incandescent and fluorescent lighting. Solid state sources generally last five to ten times longer, consume approximately 25% of the energy, are associated with lower fire risks due to lower operating temperatures, and are more durable and rugged than their incandescent and fluorescent counterparts.
Many solid state light designs are such that the illumination source is made up of an array of individual lighting elements. These lighting elements may be light emitting diodes (“LEDs”), lasers, laser diodes or other monochromatic or poly-chromatic sources. In applications where LEDs are used, they are typically selected from large batches of production LED dies. The fabrication process of LED dies results in variations in the LEDs' performance in various parameters. Accordingly, the LED dies typically undergo testing before they are used in the manufacture of solid state lights.
The testing of the LED dies typically results in a normal distribution. For example, testing of a production batch of LED dies for dominant wavelength performance generally yields a normal distribution, with 95% of the parts being within a 10 nm range. However, although the LED dies are generally tested for their individual performance against established specifications, the performance of each LED die is typically not compared relative to the performance of the other LED dies in the production batch, except for purposes of statistical analysis. Each LED die is tested against a specification, and determined to either pass or fail. After testing of the LED dies is complete, traditionally, passing LED dies are selected randomly in the manufacture of LED arrays.
Some parameters that may be tested to characterize an LED's performance may include, but are not limited to, dominant wavelength (“DW”), peak wavelength (“PW”), forward voltage (“Vf”), uniform light output, total luminous flux (“TLF”), and light color rendering index (“CRI”). Dominant wavelength is defined as the wavelength of a monochromatic stimulus that, when additively mixed in suitable proportions with the specified achromatic stimulus, matches the color stimulus considered. In other words, the DW is the single wavelength of color that is perceived from a polychromatic light source. Peak wavelength is the single wavelength at which the radiant intensity of the light source is at its maximum. Forward voltage is the voltage drop across a forward biased LED. Uniform light output is a measure of the quality of the light by measuring the uniformity of power emitted by a light source across all possible viewing angles. Total luminous flux is the human-eye-weighted sum of power emitted by a light source in the band of visible wavelengths of light. Color rendering index is another measure of the quality of a light source by measuring the proportions of all the wavelengths of visible light present in the light emitted by the source.